The present invention relates to electrical circuit protection devices such as overload relays, and more particularly to an improved and highly flexible electronic control system therefor.
Circuit protection devices such as circuit breakers, relays, contactors and the like are commonly used for disconnecting electrical circuits upon the detection of undesired currents. In addition to breaking the circuits in which the currents flow, other functions may be provided such as actuating alarms and safety devices, or the control of other apparatus in response to a sensed current characteristic. While in principle the opening of an electrical circuit in response to undesirably high currents is a simple procedure, in practice the operation of such protective devices is highly complex owing to the various, often conflicting requirements of electrical systems.
For instance, while it is necessary to protect electric motors from high currents which could damage or destroy the windings, in order to start a motor under load a high initial current is required. Also, during the operation of various electrical equipment, for instance under changing loads, high current flow must be tolerated for short periods of time. Further a single "threshold" for current flow cannot be assigned since a small overcurrent condition can be tolerated far longer than a high overcurrent condition. For these reasons industrial relays and contactors are commonly provided with complex control mechanisms which make use of two or more different current-responsive stages in an attempt to "tailor " the tripping characteristics of the device to a desired application.
In principle it is known that more sophisticated control systems can be designed which replace the present electromechanical, magnetic, and thermal controls. If cost were not a consideration a number of approaches could be implemented for deriving a measure of current flow and converting it into appropriate digital form. However for small, inexpensive control devices such as overload relays conventional means of deriving and processing signals have proven prohibitively expensive and overly complex.
Recently efforts have been made to design electronic control systems which make use of digital and other allied signal processing techniques which will provide the desired functional flexibility, and eliminate the need for mechanical adaptations to change the range or operating characteristics of a control. Two examples of such a system are shown in U.S. Pat. Nos. 4,219,585--DePuy et al and 4,219,860--DePuy. These patents disclose an overcurrent relay control which utilizes digital sampling, multiplexing and signal accumulation techniques for detecting overcurrent conditions in one or more phases of a multiphase electrical system. However, while quite flexible such systems are comparatively complex and therefore expensive. Reducing the complexity of such a control system or the sensing elements which it utilizes ordinarily results in a loss of accuracy or resolution and overall capability. It will therefore be appreciated that it would be highly desirable to provide an improved circit interrupting control whose characteristics can be varied without the need for physically interchanging or adding special elements and which utilizes relatively inexpensive signal processing apparatus.
It is therefore an object of the present invention to provide an improved sensing and control apparatus for circuit interrupters of the overload relay and contactor type.
Another object is to provide an interrupter control system which can be utilized to monitor single or plural phase electric power sources.
Another object is to provide an inexpensive solid-state control for a relay which converts a composite multiphase current envelope into digital signals to resolve monitored current characteristics to a high degree of accurancy.
Still another object is to provide an overload relay control system which emulates the thermal characteristics of a circuit being protected.
It is a further object of the invention to provide a low cost means for an overload relay or similar circuit interrupter which can monitor plural phases without the need for signal multiplexing or utilizing redundant signal processing paths.